import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world!");
    }

    //等差数列划分II - ⼦序列（hard）
    public int numberOfArithmeticSlices(int[] nums) {
        Map<Long, List<Integer>> hash = new HashMap<>();
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            long tmp = (long) nums[i];
            if (!hash.containsKey(tmp)) hash.put(tmp, new ArrayList<Integer>());
            hash.get(tmp).add(i);
        }

        int[][] dp = new int[n][n];
        int sum = 0;
        for (int j = 2; j < n; j++)
            for (int i = 1; i < j; i++) {
                long a = 2L * nums[i] - nums[j];
                if (hash.containsKey(a)) {
                    for (int k : hash.get(a))
                        if (k < i) dp[i][j] += dp[k][i] + 1;
                        else break; // ⼩优化
                }
                sum += dp[i][j];
            }
        return sum;
    }
}